After processing, such as peeling or cutting, most fruits and vegetables discolor. The discoloration is typically due to enzymes within the fruits or vegetables that metabolize colorless endogenous chemical compounds into colored chemical compounds that lead to the discoloration, often a browning. A major enzyme responsible for browning in many fruits and vegetables is polyphenoloxidase.
Various methods have been used to prevent the browning of processed produce. A very effective means of preserving produce is through the use of sulfites (e.g., U.S. Pat. No. 2,894,843 discloses the treatment of fruits and vegetables with various sulfites). Sulfites were widely used for preserving minimally processed fresh fruits and vegetables; however, due to health concerns, the US Food and Drug Administration no longer allows the use of sulfites for the preservation of fresh, including minimally processed, produce.
Since sulfites are no longer allowed for preserving fresh produce, alternative methods have been developed. A commonly used approach is treating the processed produce with ascorbic acid or an ascorbate salt (e.g., see U.S. Pat. Nos. 3,754,938 and 4,011,348). Ascorbic acid is an antioxidant and prevents the enzymatic oxidation, and coloration, of endogenous chemical compounds in the fruit or vegetable; however, the effect is short lived or requires very high concentrations of antioxidant (Gil et al., “Response of Fuji Apple Slices to Ascorbic Acid Treatments and Low-Oxygen Atmospheres,” HortScience, volume 33, number 2, 1998, pp. 305-309).
U.S. Pat. Nos. 5,925,395 and 5,939,117 disclose treating fresh cut fruit and vegetables with a combination of ascorbate (or erythorbate) ions and calcium ions. Preservation lasts several weeks; however, an excessive amount of ascorbate ion in some embodiments (up to 22.5% in the preservation solution) is needed for preservation.
U.S. Pat. Nos. 7,851,002, 8,101,221, and 8,609,169 take a similar approach of combining ascorbic acid/ascorbate (or erythorbic acid/erythorbate) with various metal ions such as magnesium, zinc, strontium, lithium, barium, aluminum, copper, iron, ammonium, manganese, and potassium. Many of these metal ions are not preferred for a food preservation product and a large concentration of preservative is required.
U.S. Pat. No. 5,922,382 discloses a method for preserving fresh cut fruit using ascorbic acid. However, the method is labor intensive and involves the use of sanitizing and flavoring agents. In addition, the concentration of ascorbic or erythorbic acid in the preservation solution is high (up to 15% weight/volume).
U.S. Pat. No. 6,749,875 (and patent application US 2002/0054950) and international patent WO 01/64041 use a combination of flavonoid and food grade antioxidant in a ratio by weight of 1:50 to 1:150. The flavonoids listed in the patent claims include acerola, quercitin, hesperidin, rutin, and flavonoid rich extracts from pine bark, grape seeds, citrus seeds, cranberries, Juniper berries and rosehips. Furthermore, the patent defines flavonoids, which is technically a very broad and general classification for a wide array of plant-derived polyphenolic compounds, as flavone, flavonols, flavanones, chalcones, anthocyanidins, proanthocyanidins, and isoflavonoids. The US patent states that the flavonoid and antioxidant can be diluted in water, tea infusion, or fruit juice for preparation of the fruit or vegetable preservation solution. The US patent discloses two formulations that include a flavonoid source, an antioxidant, and a water+tea dilution solution. For example, one formulation for treating chestnuts included 90 mg enzogenol (flavonoid), 5 g calcium ascorbate (antioxidant), and 200 ml water+5 mg green tea (dilution solution). Although green tea was included, it was at a very low concentration (0.0025 wt %) and was not taught for the purposes of supplying flavonoids since the flavonoids in green tea are primarily catechins, which are not listed as applicable flavonoids in the patent. The patent instead states that tea is used as a dilution solution. Furthermore, the patent examples required modified atmosphere packaging for packed fruit, whereas, it is desirable to develop an antibrowning preservative that does not have special atmosphere handling or packaging requirements.
It is not surprising that catechins (also known as flavanols), particularly green tea extract catechins, were not included at significant levels in the preservative formulations of U.S. Pat. No. 6,749,875 and international patent WO 01/64041, since catechins have a strong brown color and can result in significant discoloration of produce, particularly light colored flesh, such as apples, pears, and potatoes, when used as a treatment (e.g., dipping) solution. Despite the residual self-staining caused by the green tea extract catechins, there are several desirable properties of green tea extract catechins that make them particularly suited for preventing the enzymatic discoloration of produce. First, green tea extract catechins are antioxidants and can reduce the enzymatic oxidation of colorless endogenous chemical compounds within the fruit or vegetable into colored, typically brown, chemical compounds (Kim et al., “New Insights Into the Mechanisms of Polyphenols Beyond Antioxidant Properties; Lessons from the Green Tea Polyphenol, Epigallocatechin 3-Gallate,” Redox Biology, volume 2, 2014, pp. 187-195). Second, green tea extract catechins have been shown to inhibit polyphenoloxidase, which is a key enzyme involved in the production of colored chemical compounds within the fruit or vegetable (Soysal, “Effects of Green Tea Extract on Golden Delicious Apple Polyphenoloxidase and its Browning,” J. Food Biochemistry, volume 33, issue 1, 2009, pp. 134-148). The major problem though is the residual self-staining caused by the normal dark color of the green tea extract catechins and this is particularly problematic as the catechins themselves become oxidized over time since oxidation results in an even darker coloration (Wang et al., “Various Antibrowning Agents and Green Tea Extract During Processing and Storage,” J. Food Processing and Preservation, volume 27, issue 3, 2003, pp. 213-225).
Martin-Diana et al. (“Green Tea Extract as a Natural Antioxidant to Extend the Shelf-Life of Fresh-Cut Lettuce,” Innovative Food Science and Emerging Technologies, volume 9, 2008, pp. 593-603) demonstrated that green tea extract treatment of fresh-cut lettuce helped to prevent the loss of the antioxidants ascorbic acid and carotenoids. However, green tea extract increased the browning of the lettuce in a concentration-dependent manner. Wessels et al. (“Effect of Selected Plant Extracts on the Inhibition of Enzymatic Browning in Fresh-Cut Apple,” Journal of Applied Botany and Food Quality, volume 87, 2014, pp. 16-23) demonstrated that green tea extract could partially retard the browning of apple slices; however, the green tea extract demonstrated significant passive staining (i.e., residual self-staining) In line with many published reports, Wessels et al. (2014) also demonstrated that green extract had a high level of antioxidant activity. Lavelli et al (“Stability and Anti-Glycation Properties of Intermediate Moisture Apple Products Fortified with Green Tea,” Food Chemistry, volume 127, 2011, pp. 589-595) added green tea extract to intermediate moisture apple products and noted that the green tea extract darkened the apple color. Soysal (2009), cited above, demonstrated that green extract inhibited polyphenol oxidase and retarded browning; however, apple browning was only investigated over a 145 minute period and the apples had significant residual self-staining as indicated by a higher browning index immediately after treatment with the green tea extract when compared to the untreated control.
The stability of green tea extract catechins is affected by various factors, with pH being a critical component (Chen et al. “Stabilizing Effect of Ascorbic Acid on Green Tea Catechins,” J. Agri. Food Chem., volume 46, number 7, 1998, pp. 2512-2516; Zhu et al. “Stability of Green Tea Catechins,” J. of Agricultural and Food Chemistry, volume 45, issue 12, 1997, pp. 4624-4628; Li et al. “Kinetic Study of Catechin Stability: Effects of pH, Concentration, and Temperature,” J. of Agricultural and Food Chemistry, volume 60, issue 51, 2012, pp. 12531-12539). Conflicting reports have demonstrated the role of other additives on the stability of green tea extract catechins themselves. For example, Ortiz et al. (“Interaction of Environmental Moisture with Powdered Green Tea Formulations: Effect on Catechin Chemical Stability,” J. of Agricultural and Food Chemistry, volume 56, issue 11, 2008, pp. 4068-4077) demonstrated that ascorbic acid increased green tea extract catechin degradation; whereas, Chen et al. (1998) demonstrated that ascorbic acid decreased green tea extract catechin degradation. Wang et al. (2003) demonstrated that citric acid decreased the degradation of green tea extract catechins and helped to retard the additional browning of the green tea extract due to the degraded catechins.
It is an object of this invention to provide an improvement in preserving minimally processed produce using a naturally-derived preservative solution, particularly for preserving light or white colored produce flesh. Furthermore, special handling or packaging should not be required for the preserved produce.